This invention relates generally to electronic amplifiers and, in particular, to apparatus and methods for enhancing the dynamic range of multi-reference switching amplifiers.
Many electronic devices operate on the principle of providing the integral of two or more switched voltage or current levels as an analog output. In order to modulate this analog output, the effective duty cycle of the switched voltages or currents is changed by various means in accordance with an input source. In such devices, predictable monotonic resolution is attainable only at pulse widths above the minimum imposed by the aggregate propagation delays of the physical system. Below this minimum, pulse widths obtained do not correlate linearly to their commanded width.
Multiple-reference switching amplifiers enhance resolution by switching two or more reference voltages to a load. Configurations of this type are described in patent application PCT/US99/26691, entitled xe2x80x98Multi-Reference, High-Accuracy Switching Amplifier,xe2x80x99 the contents of which are incorporated herein by reference.
Although the addition of a second reference voltage improves resolution, it does not directly address resolution near zero output, where propagation delays impact crossover distortion. In many applications, such as audio amplification, dynamic range requirements often make operation below this minimum desirable. Accordingly, a need exists to extend operation of these switching devices to facilitate resolutions significantly lower than their propagation delay.
This invention is directed to apparatus and methods for adding minimum pulse widths to the coarse resolution output of a multi-reference switching amplifier, and nulling any detectable resultant differential by addition of a dynamic offset pulse width to the fine resolution output; thus extending operation to zero while presenting the imposed minimum pulse widths as a common-mode (null) signal. Although the technique is described with reference to the coarse-resolution signal, it is equally applicable to the fine-resolution signal, or both.
A preferred method according to the invention includes the steps of separating the input signal into coarse and fine resolution outputs, adding a minimum pulse width to the coarse resolution output; and adding a dynamic offset pulse width to the fine resolution output so as to null differential signals which would otherwise be present across the load.
In terms of circuitry, a data separator is employed for separating the input signal into coarse and fine resolution outputs, and a summer adds the minimum pulse width to the coarse resolution output. Pulse-width modulation converters transform the modified coarse- and fine-resolution outputs prior to presentation to the load through routing logic.